The invention relates to an apparatus for the injection molding of plastic material, comprising a mold into which molten plastic material is introduced, wherein there is provided a device for introducing liquid, in particular water, into the interior of the molten material, which includes a pump driven by a motor.
Gas injection technology has been used for about 15 years on an industrial basis for the production of injection-molded hollow bodies. The gases used (generally nitrogen) however suffer from the disadvantage that process management is more difficult by virtue of the high degree of compressibility and cooling of the molten plastic material from the cavity side is not efficient, by virtue of the low heat absorption capacity of the gases.
Those disadvantages can be overcome by using liquid, in particular water, in place of a gas. In that situation, in the simplest case, the water is injected into a tool cavity which is partially filled with molten plastic material. In the first step in the process, complete shaping of the component is effected by displacement of the molten material by way of the injected water, with the simultaneous formation of the cavity in the inner core. Thereafter, the water pressure performs the post-pressure function, in a similar manner to the situation in conventional injection molding. Usually, in the pressure-holding phase, the water is held at a pressure level at which vaporization of the water is avoided. By virtue of its substantially greater specific heat the water can also absorb a considerable part of the amount of heat from the cavity side, from the molten material. If, in dependence on component geometry, in a so-called flushing process, it is possible to produce a water through-flow, the cooling time can be reduced by up to 70%, in comparison with gas injection technology.
Process variants which are known from gas injection technology such as expelling excess molten material from the inner core of a component into a subsidiary cavity or back into the screw antechamber can also be implemented with the water injection technology.
Governed by historical development, the test installations which are known for water injection are structurally derived from the installations which are known for gas injection technology. In that case however by virtue of the high compressibility of the gas the nitrogen is almost exclusively injected in pressure-regulated mode. The time required for cavity formation is thus greatly dependent on the viscosity of the molten material and thermal boundary conditions. In the case of water in contrast, because of the low level of compressibility, outside the pressure-holding phases, control or regulation by way of the volume flow presents itself as an option.
All previously known installations involve pumps driven at a constant speed of rotation. That is not advantageous both for energy reasons and also for reason relating to wear when using a pressure-limiting valve in the by-pass (see Walter Michaeli et al, xe2x80x9cKxc3xchlzeit reduzieren mit der Wasser-Injecktionstechnikxe2x80x9d in KU Kunststoffe, volume 90 (2000) 8).
In accordance with the invention it is provided that the speed of rotation of the motor and/or the stroke or delivery volume of the pump is variable.